Erythropoesis

Erythropoesis is the process by which erythrocytes (i.e., red blood cells, RBC) are manufactured from their progenitor stem cells in bone marrow. This article also addresses their life cycle while circulating in blood and their eventual breakdown and clearance.

Like all blood cells, they begin from a pluripotent stem cell. There are stages of development purely in the marrow, and also in circulation.

Marrow development of blast cells

The first step is the proerythroblast, which is a RBC-specialized stem cell with an approximate lifetime of 16 hours. When the nucleus of this cell shrinks and the cytoplasm darkens with additional ribosomes, it becomes a basophilic erythroblast, lasting 0.8 days.

See also: Human iron metabolism

In the next stage, the process of iron storage begins with the formation of polychromatophilic erythroblasts. At this point, the cell produces hemoglobin, although it does not contain the iron reserves of a normal erythrocyte. The cell, which previously stained with basic stains such as methylene blue, now stains both blue, and red with acid stains such as eosin. This stagelasts 1.3 days.

Polychromatic erythroblasts spend about 2 days in development, and stain more uniformly with eosin.

Reticulocyte formation

Reticulocytes exist both in marrow and circulating blood. During reticulocyte formation, the nucleus disappears from the cell, but a visual "reticulum" of ribosomes remain. Typical times in marrow are 1.7 days and 1.3 in blood. Some nucleated erythroblasts may circulate, especially if there is a strong demand for blood cell production.

Circulating erythrocytes

In normal individuals, erythrocytes circulate for approximately 120 days. Mature erythrocytes are simple cells, having lost both the nucleus and reticulum of progenitors.